In many evaporation installations, degassing stages are employed for volatizing residual gasses remaining in the distillate after the main evaporation operations take place. The distillate is propelled out of the degassing stage by means of a pump whose suction inlet is connected to the output of the degassing stage and whose pressure outlet is typically connected to a cooling stage for reducing the temperature of the distillate emitted from the degassing stage for purposes of subsequent effective utilization.
The degassing stage typically has a reservoir in the bottom portion thereof for generating stripping vapor and collecting the distillate after the residual gases are expelled therefrom. Consequently, when the pump is propelling the distillate out of the degassing stage at a desired rate, a correspondingly predetermined liquid height level of the distillate in the reservoir is maintained. Since variations in the height of the column in the reservoir are indicative of variations in the rate of flow of the distillate, it has been common in the past to provide liquid height level probes to monitor the liquid height level, and regulation valves to maintain such level constant. Unfortunately, regulation schemes of the known type are susceptible to errors and disturbances. Particularly in nuclear installations it may be very expensive to meet the stringent specifications for the above mentioned devices for the control of the liquid height level.